The present invention generally relates to printing apparatus and methods and more particularly relates to a printer apparatus, and method therefor, capable of varying direction of an ink droplet therefrom for improved accuracy of ink droplet placement.
An ink jet printer produces images on a receiver medium by ejecting ink droplets onto the receiver medium in an image-wise fashion. The advantages of non-impact, low-noise, low energy use, and low cost operation in addition to the capability of the printer to print on plain paper are largely responsible for the wide acceptance of ink jet printers in the marketplace.
However, one problem associated with piezoelectric ink jet printers is placement errors of the ink droplets on the receiver medium. Such errors are due, for example, to variability in the print head manufacturing process. That is, during the print head manufacturing process, ink nozzles, which are attached to the print head, are not made identical. These manufacturing variabilities may also result in asymmetric placement of ink nozzles in a nozzle plate with respect to ink channels that otherwise should be aligned with respective ones of the nozzles. In addition, these manufacturing variabilities may result in the nozzles having non-round openings through which the ink droplets must pass. Thus, these nozzles tend to eject ink droplets in directions different from an ideal direction normal to the nozzle plate in which the nozzles are formed. Such misdirected ink droplet ejection causes misplacement of the ink droplets on the receiver medium. These ink droplet placement errors in turn produce image artifacts (i.e., defects) such as banding, reduced sharpness, extraneous ink spots, ink coalescence and color bleeding.
One method to reduce directional errors in the ejected ink droplets is to minimize the distance between the print head and the receiver medium. Minimizing distance between the print head and receiver medium minimizes error represented by the distance on the receiver medium between a correctly placed droplet and a misplaced droplet. However, a limitation of this method is that if the print head is arranged too close to the receiver medium, there is an increased risk that ink in the ink nozzles will contact the receiver medium even before ink ejection occurs. When this occurs, the ink spreads-out across the receiver medium in a uncontrolled manner to contaminate the receiver medium.
Another problem associated with ink jet printers of the piezoelectric type is so-called mechanical "cross-talk" between ink channels forming an ink jet printhead. Cross-talk between the channels interferes with precise ejection of ink droplets from neighboring channels, which in turn reduces accuracy of ink droplet placement on the receiver medium.
Techniques to improve ink droplet placement and to reduce cross-talk are known. An ink jet printhead capable of changing direction of ejected ink droplets and having negligibly low mechanical over-coupling from one channel to another is disclosed in U.S. Pat. No. 4,842,493 titled "Piezoelectric Pump" issued Jun. 27, 1989 in the name of Kenth Nilsson. This patent discloses a piezoceramic wafer into which grooves have been sawed from the upperside and underside of the wafer. The grooves on the upperside and underside of the wafer lay offset relative to one another and partially overlap. The grooves on the upperside of the wafer eject ink droplets while the grooves on the underside of the wafer, which are offset from the ink grooves on the upperside of the wafer, contain only air. In this manner, deformation of the walls of one ink groove is hardly at all transmitted to another ink groove because adjacent ink grooves are effectively separated by an intervening air-filled groove.
Moreover, U.S. Pat. No. 4,842,493 to Kenth Nilsson also discloses that direction of the ejected ink droplets can be changed with assistance of a cover which covers the ink grooves. This cover comprises a plurality of channels cut therein. A pair of the channels proceed at an acute angle relative to each of the ink grooves. Ink from an ink groove is caused to flow into a selected one of the two channels associated with each ink groove. In this manner, ink droplets depart the printhead in a direction corresponding to the acute angle of the selected channel.
However, although the Nilsson device includes a cover having channels for directing ink droplet ejection, the device disclosed in the Nilsson patent does not appear to provide for easily changing direction of ink droplet ejection as the printhead operates. That is, the channels formed in the cover of the Nilsson device are machined when the printhead is manufactured and therefore maintain their fixed acute angle during operation. A new cover must apparently be machined to replace an existing cover when change in direction of ink droplet ejection is desired. Thus, the Nilsson device appears to require disassembly of the device to vary ejection direction of ink droplets. Such a cover change-out is inconvenient and costly during field use of an ink jet printer. Thus, the Nilsson device does not appear to provide for variable change in ink droplet direction during operation. Moreover, although the Nilsson device provides for reduction in "cross-talk", the Nilsson device does not appear to provide reduction in cross-talk in combination with variable change in ink droplet direction.
Therefore, there has been a long-felt need to provide a printer apparatus, and method therefor, capable of varying direction of an ink droplet therefrom for improved accuracy of ink droplet placement.